A promoter is a specific DNA sequence that signals where RNA synthesis should begin. Most genes in higher eukaryotes are regulated by the combination of a nearby upstream promoter element plus an enhancer element. The enhancer typically works by providing binding sites for gene regulatory proteins that increase transcription.
DNA vectors (e.g. plasmids; viruses) can be modified to include a gene for a foreign protein. Some of these recombinant vectors contain a promoter/enhancer necessary for expression of the gene upstream from what is known as a cloning site (e.g. a rare or unique restriction enzyme site where a foreign gene of interest can be inserted). The recombinant expression vectors can then be transfected into mammalian cells or tissues where the foreign gene is to be expressed.
Such expression vectors are of great value for purposes such as transgenics and gene therapy. In transgenics, recombinant genes are transferred into animals at an early, embryonal stage of development where the gene inserts into the genome and is passed on to all cells of the animal. Expression of the gene then supplements defects, adds desirable traits, or helps in understanding development.
Gene therapy is slightly different. Current strategy for gene therapy is first to identify the defective gene, then to supplement the defective tissues with the functional gene. Transgenics and gene therapy are merely two examples of a broad range of uses for expression vectors in biology and medicine. Expression vectors also have many in vitro uses, such as permitting production of large amounts of a protein of interest.
Viral recombinant promoter elements are currently the most widely used. Typically they are from pathogenic viruses such as Epstein Barr Virus (EBV), Human Cytomegalovirus (CMV), Rous Sarcoma Virus (RSV) and Simian Virus 40 (SV40). See M. Manthorpe, et al., 4 Human Gene Therapy 419-431 (1993); J. Marx, 262 Science 29-30 (1993); and M. Kay, et al., 262 Science 117-119 (1993). These references and all other references cited herein are hereby incorporated by reference as if fully set forth herein. The use of viral promoters has lead to safety concerns, and in some cases certain viral promoters have turned off after a time when in a eukaryotic host. Also, some viral promoters have over promoted at first (and thus dominated the cell).
The very few mammalian promoters in use in recombinant systems have been comparatively weak in their ability to initiate transcription, and/or are restricted to a limited set of tissues, and/or are from hosts that are not desirable for various purposes. For example, a human promoter inserted into a human cell might interact in undesirable ways with other cell functions. Also, promoters and enhancers that are too different from human cell promoters might not be able to work as efficiently with certain host cells. Some known MHC promoters are reported in J. Weissman, et al., 11 Molecular and Cellular Biology 4217-4227 (1991) (pig); A. Kimura, et al., 44 Cell 261-272 (1986) (mouse); and R. Hakem, et al., 147 Journal of Immunology 2384-2390 (1991) (human).
Accordingly, there is a need for an improved mammalian promoter/enhancer that when part of a recombinant sequence initiates high levels of transcription in a wide range of mammalian cell types.